I would give it a bucket full of water twice a week between now and the end of August … poured slowly over the root area so that it soaks in and doesn’t run off … whether it rains or not.
Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.
I can't tell you what the cause is Nigel, but I can tell you I've also had Group 3 clematis looking a bit ropey in the first months after planting, but they always seem to come through looking much stronger and healthier in their 2nd year.
You were spot on. Doing well this year. Plenty of buds.
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Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.